The process of forming microlenses may be one of the most important processes influencing the performance of an image sensor. During the process for forming microlenses, gaps between the microlenses are very difficult to remove when lens patterns are formed using photolithography. It is very difficult to control the direction of light signals delivered to a lower layer through the gaps. When light passes through gaps to a lower photodiode layer, it may strike adjacent pixels, generating cross-talk, which considerably reduces the performance of the image sensor.
FIG. 1 is a view of an image sensor manufactured according to a related art. FIG. 2 is an enlarged view of a microlens unit. FIG. 3 is a view explaining a process for forming microlenses according to a related art. As shown in FIG. 1, a CMOS image sensor includes a photodiode region for receiving light using microlenses, and a logic region allowing signal transfer between layers.
A metal/insulator/metal (MIM) layer including a first metal layer 21, a dielectric layer 22, and a second metal layer 23 are formed in a logic region of a substrate 10. A third meal layer 60 is formed in a photodiode region of the substrate 10. A first interlayer insulating layer 30 is formed over the entire surfaces of the photodiode region and the logic region. The first interlayer insulating layer 30 may be etched to form a predetermined contact hole, and metal may be deposited to form a contact plug 40. A fourth metal layer 50 may be formed over the contact plug 40, so that an upper metal line may be formed by the fourth metal layer 50.
After a second interlayer insulating layer 70 is formed over the entire surfaces of the photodiode region and the logic region, a process for forming microlenses is performed over the photodiode region. After the second interlayer insulating layer 70 is etched to a predetermined thickness, a light receiving portion, that is, a color filter layer 90, a planar layer 91, and a lens layer 92 are formed. In particular, after the color filter layer 90 is formed, the planar layer 91 is formed over the color filter layer 90, and a process for forming the microlenses is invoked. The process for forming the microlenses uses a reflow process using a photoresist formed over the planar layer 91.
FIG. 2 is an enlarged view of the light receiving portion including the color filter layer, the planar layer, and the lens layer. The planar layer 91 is formed over the color filter layer 90 which transmits light having a predetermined wavelength. Microlenses forming the lens layer 92 are formed with a predetermined gap over the planar layer 91. The direction of light delivered to a lower portion through the gap is very difficult to control. Light travelling through the gaps may erroneously propagate to adjacent pixels of the color filter layer 90 as illustrated.
A method for manufacturing the planar layer will be described with reference to FIG. 3. In an image sensor in the related art, open photoresist patterns may be formed on a peripheral circuit region, not a pixel region. A mask with a transparent portion over the pixel region is used to expose the photoresist.
The portion of the photoresist that has received light remains, and a portion of the photoresist that has not received light is removed, so that a planar layer 91 is manufactured as illustrated in FIG. 4. The above-described photoresist is also called a negative photoresist.
As described above, since the microlenses are formed over a planar layer having an upper planar surface. The microlenses are formed with a predetermined gap, so propagating light may deflect to adjacent pixels of the color filter layer, which reduces the operating performance of the image sensor.